1. Field of the Invention
The present invention relates to a method of coupling optical parts and refractive-index imaging material, particularly to a method for coupling optical parts such as optical waveguides and optical devices in optical communication and optical interconnection and to a material used for coupling the optical parts.
2. Description of the Related Art
Optical parts are used for various optical information processing systems including optical interconnection and optical communication. For example, light emitted from a light source such as a semiconductor laser is transmitted through a waveguide or optical fiber and the transmitted light is changed into electric signals by a photodiode.
A high coupling efficiency is required for optical coupling between optical parts. To obtain a high coupling efficiency, it is necessary to completely align optical electromagnetic fields between optical parts. Therefore, it is necessary to equalize the diameters of optical coupling faces at a joint between the optical parts and minimize optical axis misalignment and angular misalignment of the optical parts. To realize this, it is necessary to accurately align the coupling faces between optical parts. However, it is not easy to improve the alignment accuracy. Therefore, a method for easily and efficiently optically-coupling optical parts such as optical fibers and optical devices is desired.
FIGS. 1 (a) to 1 (c) show a general step for optical-coupling two optical fibers.
These optical fibers 1 and 2 have a structure in which cores 1a and 2a are enclosed by cladding 1b and 2b and the cores 1a and 2a have larger refractive index than the cladding 1b and 2b.
To optically couple the optical fibers 1 and 2, as shown in FIG. 1 (a), the optical fiber 2 is secured to a fiber securing portion 3 and the optical fiber 1 is set to a mobile stage 4. Then, an operator moves the mobile stage 4 while observing it with a microscope 5 to accurately align the edge of the core 2a of the optical fiber 2 with that of the core 1a of the optical fiber 1 as shown in FIG. 1 (b). After the alignment is completed, the joint between the optical fibers 1 and 2 is welded with an arc discharge apparatus 6.
However, the above method for optically coupling optical parts has a problem that optical coupling between optical parts cannot easily be performed because it is necessary to previously perform accurate alignment.
A measure for solving the above problem is desired for optical coupling between optical fibers or a coupler for optically coupling a light-emitting device or light-detecting device with an optical fiber or optical waveguide.
To accurately couple optical parts, Japanese Patent Laid-Open Nos. Sho. 53-108452 and Sho. 64-6909 disclose a method for melting the edges of two optical fibers and coupling them by the surface tension. However, these prior art references only disclose the coupling between optical fibers but do not disclose the coupling between other optical parts.
Moreover, accurate optical coupling between optical parts is considered by using couplers.
These couplers are described in the following documents.
1! Optical Communication Device Optics-Light-Emitting and Light-Detecting Devices, Hiroo Yonezu, Kogakutosho, Japan PA0 2! Optical Fiber Technology In ISDN Age, Katsuhiko Okubo, Rikogakusha, Japan PA0 3! Optical Communication Handbook, Edited by Hiroshi Hirayama et al., Kagakushinbunsha, Japan
These documents show that an edge emitting laser has a rectangular structure with an active layer of approximately hundreds of nanometers by several microns and its radiation angle ranges between 20.degree. and 60.degree. in the vertical direction and between 5.degree. and 20.degree. in the horizontal direction. A surface emitting LED has a large emitting region diameter of 30 to 40 .mu.m and a radiation angle of approximately 120.degree..
In this connection, a single-mode optical fiber has a core diameter of several to 10 microns and a multiple-mode optical fiber has a core diameter of several tens of microns. Therefore, to couple an optical semiconductor device with an optical fiber, accurate alignment along the order of 1-micron is desirable to decrease the coupling loss.
When accurately aligning and directly coupling a light-emitting device with an optical fiber by contacting the edge of the device with that of the fiber, more specifically, for direct coupling of an edge emitting laser with a single-mode optical fiber, the coupling efficiency approaches 30%. For direct coupling of the edge emitting laser with a multiple-mode optical fiber, the coupling efficiency approaches 50%. For direct coupling of a surface emitting LED with the multiple-mode optical fiber, the coupling efficiency approaches approximately 6%.
A method for setting a lens between an edge emitting laser and single-mode optical fiber has been proposed as a method for coupling the laser with the fiber. In this case, the coupling efficiency is approximately 50%. However, optical coupling becomes further difficult because the number of parts requiring accurate alignment increases.
For direct coupling of an optical fiber with a waveguide, a coupling efficiency of 56 to 79% is obtained by equalizing the core diameter of the waveguide edge diameter with that of the optical fiber and preventing misalignment of axes.
However, there is a problem that the direct coupling of the waveguide with the optical fiber is not easy because the core diameters of the waveguide and optical fiber are limited and accurate alignment at 1 .mu.m order is desired.
Moreover, a method different from the above direct coupling and lens coupling methods is proposed. In Japanese Patent Laid-Open Nos. Sho. 55-43538 and Sho. 60-173508, it is proposed to use a material whose refractive index changes by applying light to the material.
However, the optical coupler connection method proposed in Laid-Open No. Sho. 55-43538 includes a method for manufacturing an optical coupler characterized by applying light to the optical coupler substrate made of a material whose refractive index changes proportionally to a light intensity from a position where light should be inputted or outputted and changing the refractive index of the optical coupler substrate so as to form an optical waveguide in self-alignment. To use the optical coupler, it is necessary to arrange and secure optical parts including optical fibers to be coupled by the optical coupler. Therefore, for example, a hole for inserting an optical fiber is formed on the optical coupler.
In Laid-Open No. Sho. 60-173508, an optical waveguide connection method is proposed which is characterized by setting a phase-change-type photosensitive medium material between two waveguides facing each other, applying light to the photosensitive medium material from the both waveguides, and locally denaturalizing the photosensitive medium so as to form a waveguide for optical coupling.
This reference proposes to set a photosensitive medium material between waveguides to be mutually connected, form a waveguide for optical coupling in it, and thereby decrease a loss due to misalignment of optical axes and it is preferable to set the interval between waveguides to 0.1 mm or less and use ultraviolet rays as the light to be applied to the photosensitive medium material. As a result, though the light spreads due to diffraction in the photosensitive medium material, a waveguide for coupling with a small spread is formed. The optical coupling disclosed in this reference uses connection between waveguides formed in the photosensitive medium and misalignment between waveguides to be connected is also taken over between waveguides in the photosensitive medium. Therefore, it is impossible to correspond to a large misalignment exceeding a core diameter.
The following materials can be used for the optical coupler.
For example, gazette Laid-Open No. Sho. 55-43538 mentioned above discloses that a chalcogenide-based amorphous semiconductor or macromolecular material containing photopolymerizable monomer is used for the optical coupler and Laid-Open No. Sho. 60-173508 discloses that the photopolymer made by DU PONT LIMITED, Photoresist KPR (trade name) made by KODAK LIMITED, and U.V. 57 (trade name) made by OPTION CHEMICAL LIMITED are known.
The following are refractive-index imaging materials whose refractive indexes change by applying light to them.
For example, Japanese Patent Laid-Open No. Hei. 2-3081 discloses a material made of thermoplastic polymers, ethylene-based unsaturated monomers, and polymerization initiator. Japanese Patent Laid-Open No. Hei. 2-3082 discloses a material made of interpolymers containing such segments as polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, and polyvinyl formal as the main part, or made of polymerizable binder selected among groups made of the mixture of the segments, ethylene-based unsaturated monomers, and an optical initiator. Japanese Patent Laid-Open No. Hei. 3-50588 discloses a material made of solvent-soluble fluorine-contained polymerizable binder, ethylene-based unsaturated monomers, and a photopolymerization initiator. Moreover, Japanese Patent Laid-Open No. Hei. 3-36582 discloses a material made of allyl diglycol carbonate, 2,2,-bis{3,5-dibromo-4-(2-mathasryroiloxiethoxy) phenyl} propane, and a photopolymerization initiator.
However, these materials have a low heat resistance because they use thermoplastic resin and methacryroil-based polymeric products as a binder.